Change ip6_rt_cache_alloc to take a fib6_result over a fib6_info.

Since ip6_rt_cache_alloc is only the caller, update the
rt6_is_gw_or_nonexthop helper to take fib6_result.
Signed-off-by: David Ahern <dsahern@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>